Lafon disclosed modafinil and other similar compounds in U.S. Pat. No. 4,177,290 as having pharmaceutical activity on the central nervous system. In a typical prior art process, benhydrylthioacetic acid is halogenated with thionyl chloride. The chloride is then converted to the amide in methylene chloride with ammonia. The amide is then oxidized with hydrogen peroxide to provide benzhydrylsulphinylacetamide. Other derivatives of modafinil as well as methods of preparation and purification are disclosed in U.S. Pat. No. 4,127,722. However, the amide appears to be the compound of choice among the many derivatives now known.
Interest in the Lafon compounds has increased in recent years because these compounds have been discovered to have beneficial effects in the treatment of a wide variety of diseases in mammals including humans. Although first noted as a treatment for narcolepsy, more recent patents and technical publications have listed such compounds as beneficial in the treatment of Parkinson's disease, urinary incontinence, Alzheimer's disorder, ischemia and stroke. As the use of these compounds increased so has the demand for greater volumes while maintaining the highest state of purity and also avoiding process chemicals of high environmental risk.
Lafon also disclosed the use of an intermediate, a racemic mixture of benzhydrylsulphinylacetic acid to prepare an isomer of modafinil in U.S. Pat. No. 4,927,855, referring to French patent 2,326,181B. The racemic mixture of the acid was reacted with the (−)-α-methylbenzylamine to provide the (−)-benzhydrylsulphinylacetate of (−)-methylbenzylamine which is acidified with concentrated hydrochloric acid to provide (−)benzyhydrylsulphinylacetic acid. The levo-acid thus produced was then reacted with methylsulfate in water and sodium bicarbonate to provide methyl(−)benzhydrylsulphinylacetate. The acetate was then amidated with ammonia gas to provide (−)benzhydrylsulphinylacetamide.
Numerous substituted thioacetamides are disclosed in U.S. Pat. No. 6,492,396 to Bacon et al. In one synthesis scheme benzhydrol is converted to a benzhydrylthiol by reaction with thiourea that is then converted by hydrolysis to a thiouronium moiety. Subsequently, the thiouronium is converted to an acid with chloroacetic acid. The benzhydrylthioacetic acid is treated in various ways depending upon the desired derivative. To prepare the amide the acid is reacted with ammonia or an appropriate amine in an organic solvent such as tetrahydrofuran or methylene chloride. Other thioacetamide derivatives are obtained by employing N-methylmorpholine and a thioacetic acid in dimethylformamide (DMF).
A procedure for the preparation of an acetamide intermediate for the production of modafinil is disclosed in published US application 2002/0183552. According to this application a three-step procedure for preparing modafinil is disclosed starting with benzhydrol (diphenylmethanol) that is employed to prepare the benzhydrylthiocarboxamidine salt by reaction with thiourea in hydrogen bromide. The bromide salt is then reacted with chloroacetamide in aqueous sodium hydroxide to produce diphenylmethylthioacetamide. The acetamide may then be oxidized by conventional means to produce modafinil. Typically, the oxidation is provided by a reaction with hydrogen peroxide in glacial acetic acid.
Because of the growing demand for large quantities of modafinil in a highly pure state there is needed a more efficient process for preparing the product. In particular, the production of highly pure acetamide intermediate is needed to improve overall process economics.